The invention relates to a system for dissipating electrostatic charges of a pluggable electrical unit when it is brought into connection with an enclosure connected to earth potential of an electrical base unit.
Complex electrical engineering systems, in particular in measuring and control technology and in telecommunications, are often of a modular construction. This involves a large number of electrical units being accommodated in an enclosure of an electrical base unit in a pluggable manner. In accordance with the relevant regulations, the enclosure is appropriately earthed.
When a complex system of this type is expanded by adding further pluggable electrical units while operation is in progress or when one of the pluggable electrical units is exchanged, there is the risk of electrical components of the pluggable electrical unit being damaged or destroyed as a result of potential-equalizing currents induced on the electrical unit to be inserted by electrostatic charges.
It is known from DE 40 36 081 to provide the plug-in connection between the pluggable electrical unit and the electrical base unit with a leading contact, by means of which the circuit frame is led. Metallic components of the pluggable electrical unit which carry electrostatic charges are optionally electrically connected to the leading contact through a resistor, or a capacitor, or a series connection comprising a resistor and a capacitor, or back-to-back Zener diodes or a series connection comprising a resistor and back-to-back Zener diodes. In this case, the direct inclusion of the current path of the circuit frame into the discharge circuit of the electrostatic charge is regarded as disadvantageous.
In DE 297 10 574 a subrack for pluggable printed-board assemblies is provided. A circuit board of the pluggable printed-board assembly has in the edge region a strip-shaped sliding contact. The subrack is provided with earthed contact elements for establishing the electrical contact of the strip-shaped sliding contact. The front plate of the pluggable printed-board assembly is connected to the strip-shaped sliding contact through a resistor. A plurality of earthed contact elements are intended to mitigate the xe2x80x9cantenna effectxe2x80x9d of the strip-shaped sliding contact.
The present invention is based on the object of dissipating electrostatic charges on a pluggable electrical unit in such a way that disruptions to units already in operation and damage to the unit to be inserted are avoided.
According to the invention, this object is achieved by the means of Patent claim 1. Advantageous configurations of the invention are given in the dependent claims.
The essence of the invention is to lead the frame potential and the housing potential of the base unit and of the pluggable electrical unit separately and in an earth-free manner in each case and to connect them to each other and to earth potential through attenuators.
To be specific, the base unit is provided with a first and a second attenuator. The at least partially conductive enclosure of the base unit is connected to earth potential through the first attenuator. The enclosure is connected to the frame potential of the base unit through the second attenuator. The enclosure is connected to a first contact device and the frame potential of the base unit is connected to a second contact device.
The pluggable electrical unit has a third and a fourth attenuator. The housing of the pluggable electrical unit is connected to the frame potential of the pluggable electrical unit through the third attenuator and is connected to a first mating contact device through the fourth attenuator. The frame potential of the pluggable electrical unit is connected to a second mating contact device.
The first contact device of the base unit can be connected to the first mating contact device of the pluggable electrical unit and the second contact device of the base unit can be connected to the second mating contact device of the pluggable electrical unit. In this case, the first contact device and the first mating contact device are designed as a pair of leading contacts.
To describe the mode of operation, a potential difference induced by electrostatic charging between the earth potential and the housing of the pluggable electrical unit is assumed. When the pluggable electrical unit is inserted into the base unit, initially the first contact device and the first mating contact device of the pair of leading contacts are brought into connection. As a consequence of this, a flow path is formed through the fourth attenuator for attenuated potential equalization between the enclosure of the base unit and the housing of the pluggable electrical unit. In addition, the electrostatic charge on the housing of the pluggable electrical unit is dissipated to earth potential through the series connection of the fourth attenuator and first attenuator.
This advantageously realizes both the potential equalization between the enclosure of the base unit and the housing of the pluggable electrical unit and also the dissipation of the electrostatic charge on the housing of the pluggable electrical unit to earth potential without any surges and avoiding current peaks. In addition, the circuit frame and also the supply and data lines of the base unit and of the pluggable electrical unit remain unaffected by the discharge current. Consequently, any impairment or destruction of units already in operation is avoided.
As the inserting operation proceeds, the second contact device of the base unit is brought into contact with the second mating contact device of the pluggable electrical unit. In this case, the circuit frame of the pluggable electrical unit is connected to the circuit frame of the base unit. In this case, the series connection comprising the second and third attenuators is connected in parallel with the fourth attenuator, with the result that the discharge impedance of the housing of the pluggable electrical unit to earth potential in the inserted state is additionally reduced.
According to one configurational feature of the invention, it is provided that the first attenuator of the base unit comprises the parallel connection of a capacitor and a resistor. In this way, dynamic potential differences between the enclosure of the base unit and earth potential are advantageously reduced via the capacitor with low impedance. Static potential differences are balanced out via the resistor.
According to a further feature of the invention, the second attenuator of the base unit comprises the parallel connection of a capacitor and a resistor. In this way, dynamic potential differences between the enclosure and the frame potential of the base unit are advantageously reduced via the capacitor with low impedance. Static potential differences are balanced out via the resistor.
According to a further feature of the invention, the third attenuator of the pluggable electrical unit comprises the parallel connection of a capacitor and a resistor. In this way, dynamic potential differences between the housing and the frame potential of the pluggable electrical unit are advantageously reduced via the capacitor with low impedance. Static potential differences are balanced out via the resistor.
According to a further feature of the invention, the fourth attenuator of the pluggable electrical unit comprises a resistor. In this way, all the potential differences during the insertion are advantageously dissipated from the housing with a defined impedance.
According to a further feature of the invention, the pair of leading contacts is configured as a sliding contact device with a sliding contact and a slideway. In this case, the sliding contact is formed onto the conductive enclosure of the base unit. The slideway is arranged in the edge region of the pluggable unit in the plugging direction and is dimensioned in its length such that the pair of leading contacts is closed during the insertion and open in the fully plugged state of the pluggable unit.
In this way, during the insertion phase, the intended potential equalization between the enclosure of the base unit and the housing of the pluggable unit is temporarily brought about via the pair of leading contacts and the fourth attenuator and, in the inserted state, is permanently maintained via the connected contacts for the frame potential and the second and third attenuators. In the inserted state of the pluggable unit, coupled-in electromagnetic interference received via the slideway is discharged to frame potential via the third and fourth attenuators. The coupled-in interference is in this case attenuated to such a degree that functional disruptions to units in operation are avoided.